This invention relates in general to motor vehicle hydraulic brake systems, and in particular to brake master cylinders.
Vehicles having a hydraulic brake system generally have a master cylinder which pressurizes hydraulic fluid within a hydraulic brake circuit to operate the brakes of the vehicle. A master cylinder generally includes a body having a cylindrical bore formed therein. Typically, a master cylinder will contain two (or more) pistons within the cylindrical bore, each pressurizing a separate brake hydraulic circuit connected to certain ones of the brakes on a vehicle, thereby decreasing the possibility of total brake system failure. In a typical tandem (two piston) master cylinder, a primary piston and a secondary piston are disposed within the cylindrical bore. A braking force is input from the brake pedal, through a brake booster if provided, through a rod entering a first axial end of the cylindrical bore which acts against the primary piston. The force is selectively applied to urge the primary piston to pressurize the hydraulic brake fluid in a primary pressure chamber. The primary chamber is in fluid communication with the primary brake hydraulic circuit. A secondary piston forms one wall of the primary pressure chamber, and is thus acted upon by the pressure in the primary pressure chamber. The secondary piston is thus urged by the pressure in the primary pressure chamber to pressurize the hydraulic brake fluid in a secondary pressure chamber. The secondary chamber is in fluid communication with the secondary brake hydraulic circuit.
If the cylindrical bore is formed as a "blind hole", the second axial end of the cylindrical bore is sealed by a portion of the master cylinder body, forming a pressure boundary for the secondary pressure chamber. However, the use of a blind hole can make it somewhat difficult to assemble the internal components of the master cylinder in the cylindrical bore because various shoulders or circumferential grooves may be required to house various components, such as seals.
In static seal master cylinder designs, the seals are held stationary relative to the master cylinder body, and slidingly seal against the movable pistons. Static seals preferably are axially supported within the cylindrical bore to hold them stationary. Although grooves to axially support such static seals can be machined into the wall of the cylindrical bore, this process is generally undesirable due to cost and manufacturing time factors.
To avoid expensive and complex machining operations for forming circumferential grooves within the cylindrical bore for mounting the static seals, it is known to assemble the secondary and primary seals into the bore along with precision machined sleeves which provide the axial support for the seals. These sleeves must be machined with precision to provide free movement and support to the pistons moving therein, and to properly position the seals. The use of sleeves generally increases the cost and the complexity of such master cylinders.
It is also known to form the cylindrical bore as a through bore so as to facilitate machining and assembly of components into the master cylinder. In this design, the second axial end of the cylindrical bore has in the past been sealed with a removable plug. Some prior art master cylinders use a removable plug having an external threaded portion which is threadably engaged with a mating internal threaded portion formed into the second axial end of the cylindrical bore of the body. However, threaded fasteners can cause contamination of the brake fluid because of small metallic particles fracturing from the threaded portions. Additionally, expensive sensitive torque equipment is generally needed to install such threaded plugs. Such removable plug designs have experienced problems with leakage past the plug, and generally increase the axial length of the master cylinder body to provide sufficient material therein to permit machining the threads into the master cylinder body. The length of the master cylinder has in recent years been an important factor in the design of master cylinders because of the tight space constraints of the engine compartment of a vehicle.
Some prior art master cylinders use a caged spring assembly positioned between the primary and secondary pistons. The caged spring assembly generally are caged to a set critical axial length dimension to correctly position the secondary piston relative to a compensation port. The compensation port is a small diameter passageway through the wall of the piston which communicates with a passage through the master cylinder body to the reservoir. The dimension of the axial length of the spring assembly is critical because the primary and secondary pistons have to be precisely positioned in order to allow fluid communication between the reservoir and the primary and secondary pressure chambers, when the master cylinder is inactive. The axial length dimension also has a direct effect on the piston stroke and lost pedal travel.
The caging of the spring is usually done with a circlip or a shoulder screw, and the set axial length dimension is established through a cooperation of several parts, which can result in a large undesirable variation because of the stack-up of tolerances. Another method of assembling the spring cage is to use an axial pin having a threaded end which is threaded into the primary piston a desired distance to set the overall length of the spring assembly. However, the assembly mechanisms which thread the pin onto the primary piston generally are closely monitored during the assembly operation. In prior art designs in which a pin was screwed into the primary piston via a spiral thread, an elaborate structure was required to fix one of the primary piston and the pin, and to rotate (or permit the rotation of) the other of the primary piston and the pin, while simultaneously advancing the two components together. In addition to being relatively elaborate, and therefore expensive, such an assembly method was relatively inaccurate since the final distance of the pin from the piston depends on predicting how far the pin would be screwed in while stopping the relative rotational movement of the pin and piston, and variations in friction encountered during the coast down period after the rotational drive is shut off. Additionally, damage to the component which is secured against rotation could occur if the component slips within its restraints.